Mixed refrigerant cycle for ethylene recovery

ABSTRACT

A closed-loop mixed refrigerant cycle provides efficient low-level refrigeration for recovering ethylene from a mixed gas feed. Compressed mixed refrigerant vapor is condensed at -20° F. to -50° F. and is subcooled to -175° F. to -225° F. by indirect heat exchange with cold H 2 , methane, and expander streams from elsewhere in the ethylene plant. A portion of the subcooled refrigerant may be flashed to provide additional subcooling of the main mixed refrigerant stream. Subcooled mixed refrigerant is subsequently flashed to provide very low temperature level refrigeration for feed condensation and demethanizer overhead condenser duties. The invention allows advantageous operation of the feed chilling train at pressures between 150 and 400 psia.

This is a Continuation-in-Part of Ser. No. 08/192,024 filed Feb. 4,1994.

FIELD OF THE INVENTION

This invention pertains to the recovery of ethylene from light gases atlow temperature, and in particular to an improved mixed refrigerantcycle to provide more efficient refrigeration for such recovery.

BACKGROUND OF THE INVENTION

The recovery of ethylene from crude light hydrocarbon gas mixtures is aneconomically important but highly energy intensive process. Cryogenicseparation methods are commonly used which require large amounts ofrefrigeration at low temperatures, and the continuing development ofmethods to reduce net power to provide this refrigeration is importantin the petrochemical industry.

Ethylene is recovered from light gas mixtures such as cracked gas fromhydrocarbon crackers which contain various concentrations of hydrogen,methane, ethane, ethylene, propane, propylene, and minor amounts ofhigher hydrocarbons, nitrogen, and other trace components. Refrigerationfor condensing and fractionating such mixtures is commonly provided atsuccessively lower temperature levels by ambient cooling water, closedcycle propylene and ethylene systems, and work expansion orJoule-Thomson expansion of pressurized light gases produced in theseparation process. Numerous designs have been developed over the yearsusing these types of refrigeration as characterized in representativeU.S. Pat. 3,675,435, 4,002,042, 4,163,652, 4,629,484, 4,900,347, and5,035,732.

The use of closed cycle mixed refrigerant systems can be integrated withone or more of the above-mentioned refrigeration methods to improve theoverall energy efficiency of ethylene recovery. Mixed refrigerants forsuch systems typically comprise methane, ethane, ethylene, propane,propylene, and optionally other light components. Mixed refrigerantsexhibit the desirable property of condensing over a range oftemperatures, which allows the design of heat exchange systems which arethermodynamically more efficient than single refrigerant systems.

U.S. Pat. No. 4,072,485 describes a mixed refrigerant cycle forproviding low level refrigeration in a natural gas processing plant, orin the cryogenic section of an ethylene plant which utilizes one or morepartial condensation stages to cool the feed gas. In this cycle, themixed refrigerant is partially condensed with cooling water or air atnear ambient temperature and then totally condensed at +50° F. andsubcooled with several levels of propane or propylene refrigeration. Inethylene plant service, the mixed refrigerant is then utilized toprovide refrigeration over the temperature range of -40° F. to -148° F.;i.e., it is confined to the same temperature range as the ethylenerefrigeration it replaces. A more specific example of this cycle forethylene plant service is described in an article by Victor Kaiser, etal., "Mixed Refrigerant for Ethylene," in the Oct. 1976 issue ofHydrocarbon Processing, pages 129-131.

U.S. Pat. 4,720,293 describes a process for recovering ethylene fromrefinery off-gas which utilizes a mixed refrigerant cycle. In thisprocess, the mixed refrigerant is utilized in a single heat exchangerover a relatively warm temperature range of +60° F. to -85° F.Refrigeration at lower temperature levels is supplied by vaporization ofseparated ethane at low partial pressure and high total pressure, and bywork expansion of light gases which are typically rejected to fuel alongwith the ethane.

With the conventional process technology described above, the feed gaschilling and demethanizing must be carried out at pressures in the rangeof 450 to 650 psia in order to achieve high ethylene recovery (994% ormore) because the propylene/ethylene cascade system can providerefrigeration no colder than -150° F. for feed gas chilling and fordemethanizer column condenser refrigeration. The amount of refrigerationfor feed cooling below -150° F. which can be produced from other processstreams in an ethylene plant is limited by operating constraints such asthe amount of high pressure hydrogen recovered and the fuel systempressure(s). These constraints limit the amount of expanderrefrigeration which can be produced, which in turn limits the ethylenerecovery. Pressures between 450 and 650 psia are required in the feedgas chilling train and in the demethanizer column so that most of theethylene can be condensed above -150° F., and so that sufficient fuelgas expansion refrigeration at colder temperatures is available tocondense most of the remaining ethylene and achieve low ethylene loss inthe demethanizer column overhead vapor.

The integration of improved mixed refrigerant cycles with conventionalintermediate and low temperature refrigeration holds promise for furtherreduction of energy consumption in ethylene recovery. In particular, itis desirable to improve the efficiency of refrigeration at the lowesttemperature levels required for high ethylene recovery. The inventiondescribed in the following specification and defined in the appendedclaims provides an improved mixed refrigeration cycle which isparticularly advantageous for ethylene recoveries of greater than 99%.

SUMMARY OF THE INVENTION

The recovery of ethylene from a feed gas containing ethylene, hydrogen,and C₁ to C₃ hydrocarbons includes the steps of compressing the feedgas, cooling the compressed feed gas to condense a portion thereof,fractionating the condensed feed gas liquids in one or more demethanizercolumns to recover a light overhead product comprising chiefly hydrogenand methane, and fractionating the demethanizer column bottoms stream torecover an ethylene product and streams containing C₂ and heavierhydrocarbons. Typically at least a portion of the hydrogen-methane vaporstream from the final ethylene recovery heat exchanger is sent to ahydrogen recovery section to produce a high-purity hydrogen product andone or more methane-rich streams.

Refrigeration for ethylene recovery is provided in an improved cycle ofthe present invention which comprises compressing a mixed refrigerantvapor containing two or more components selected from the groupconsisting of methane, ethane, ethylene, propane, and propylene, andcooling the resulting compressed vapor to yield a condensed mixedrefrigerant stream. The condensed mixed refrigerant stream is subcooledby indirect heat exchange with one or more cold process streams to yielda subcooled mixed refrigerant. A first portion of the subcooled mixedrefrigerant is flashed and used to provide overhead condenserrefrigeration by indirect heat exchange for at least one of thedemethanizer columns, which warms and at least partially vaporizes thefirst portion of subcooled mixed refrigerant. A second portion of thesubcooled mixed refrigerant is flashed and the resulting refrigerantprovides at least a portion of the refrigeration required for thecooling and partial condensing of the feed gas by indirect heatexchange, which warms and at least partially vaporizes the secondportion of subcooled mixed refrigerant. Preferably the feed gas coolingand condensing are accomplished in one or more dephlegmators, butalternatively one or more partial condensers can be utilized. Theresulting warmed vapors of the first and second portions of mixedrefrigerant are combined and compressed to complete the refrigerationcycle.

One of the cold process streams for mixed refrigerant subcooling may beprovided by flashing and vaporizing a third portion of the subcooledmixed refrigerant, and the warmed vapor is combined with the first andsecond portions of mixed refrigerant to provide mixed refrigerant vaporfor compression to complete the refrigeration cycle. Another one or moreof the cold process streams can be provided by work expanding the lightoverhead product from the one or more demethanizer columns and/or anyhydrogen and methane which is not processed in the hydrogen recoverysection. Additional cold process streams such as the hydrogen andmethane-rich streams from the hydrogen recovery section also can be usedfor mixed refrigerant subcooling.

In an alternative embodiment of the invention, subcooled mixedrefrigerant is provided for two feed condensing zones in series in whichpartially vaporized mixed refrigerant from a cold feed condensing zoneprovides a portion of the refrigeration to a warm feed condensing zoneto provide two condensed feed liquid streams for further separation.Preferably, the two condensing zones utilize dephlegmators, butalternatively can utilize partial condensers. Combinations of partialcondensers and dephlegmators may be used; more than two feed condensingzones in series can be used if desired.

A key feature of the present invention is that the feed chilling trainand the downstream separation equipment can be operated advantageouslyin the pressure range of 150 to 400 psia, thereby achieving satisfactoryethylene recovery with lower refrigeration requirements and lowercapital costs in the downstream separation equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of one embodiment of the closed loopmixed refrigerant cycle of the present invention.

FIG. 2 is a schematic flow diagram of an alternative embodiment of theclosed loop mixed refrigerant cycle of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a typical ethylene recovery process, a feed gas comprising hydrogen,methane, ethane, ethylene, propane, propylene, and minor amounts ofother light components is compressed, cooled, and partially condensed insingle stage condensers or alternatively in one or more dephlegmatorswhich impart several stages of separation during the condensation step.The condensate is separated from lighter gases and is passed to one ormore demethanizer columns which recover a light gas overhead comprisingchiefly methane and hydrogen, and a bottoms stream rich in C₂ and C₃hydrocarbons. This hydrocarbon stream is further fractionated to yield ahigh purity ethylene product, an ethane-rich byproduct, and a stream ofC₃ and heavier hydrocarbons.

Essentially all ethylene plants use an ethylene-propylene cascaderefrigeration system to provide the major portion of refrigerationrequired in the ethylene plant. Most of the propylene (high level)refrigeration is utilized at several pressure/temperature levels in theinitial feed precooling and fractionation sections of the plant to coolthe feed from ambient temperature to about -35° F. and to condense theethylene refrigerant at about -30° F. Similarly, the ethylene (lowlevel) refrigeration is utilized at several pressure/temperature levelsin the cryogenic section of the plant to cool the feed from -35° F. toabout -145° F. in order to condense the bulk of the ethylene in the formof liquid feeds to a demethanizer column, and in the demethanizer columnoverhead condenser at about -150° F. to provide reflux to that column.Ethylene is normally not used to provide refrigeration below -150° F.since that would result in sub-atmospheric pressure at the suction ofthe ethylene compressor. Refrigeration below -150° F., to condense theremaining ethylene from the feed, is provided primarily by workexpansion of rejected light gases, H₂ and methane, and/or byvaporization of methane refrigerant which has been condensed by ethylenerefrigerant. The work expanded gases are normally used as fuel andconsist primarily of the overhead vapor from the demethanizer column,mostly methane, and any uncondensed feed gas, mostly H₂ and methane,which is not processed in the H₂ recovery section of the ethylene plant.

With the conventional process technology described above, the feed gaschilling and demethanizing must be carried out at pressures in the rangeof 450 to 650 psia in order to achieve high ethylene recovery (99% ormore) because the propylene/ethylene cascade system can providerefrigeration no colder than -150° F. for feed gas chilling and fordemethanizer column condenser refrigeration. The amount of refrigerationfor feed cooling below -150° F. which can be produced from other processstreams in an ethylene plant is limited by operating constraints such asthe amount of high pressure hydrogen recovered and the fuel systempressure(s). These constraints limit the amount of expanderrefrigeration which can be produced, which in turn limits the ethylenerecovery. Pressures between 450 and 650 psia are required in the feedgas chilling train and in the demethanizer column so that most of theethylene can be condensed above -150° F., and so that sufficient fuelgas expansion refrigeration at colder temperatures is available tocondense most of the remaining ethylene and achieve low ethylene loss inthe demethanizer column overhead vapor.

The present invention utilizes an improved closed-loop mixed refrigerantcycle which provides efficient low-level refrigeration by using therefrigerant in a subcooled state for the demethanizer overhead condenserduty and for feed cooling and condensation. The mixed refrigerant,consisting predominantly of methane, ethane or ethylene and propane orpropylene, at a temperature of about -30° F. to -60° F. and 15 to 50psia (preferably 20 to 35 psia), is compressed to 250 to 500 psia(preferably 300 to 450 psia) and cooled to ambient temperature withcooling water or air with essentially no condensation. The mixedrefrigerant vapor is then cooled to about -20° F. to -50° F. usingmultiple levels of propane or propylene refrigerant to condense at least80%, and preferably all, of the mixed refrigerant stream. The mixedrefrigerant liquid, and vapor if any, is then subcooled to -175° F. to-225° F., with the predominant amount of refrigeration provided by coldH.sub. 2 and methane streams returning from the H₂ recovery section ofthe ethylene plant and by expanded light gases from the overhead of thedemethanizer column and/or uncondensed feed gas which is not processedin the H₂ recovery section. These streams will typically be in the rangeof -175° F. to -235° F. entering the mixed refrigerant subcooler andwill be warmed as much as possible for maximum refrigeration recovery. Aportion of the subcooled mixed refrigerant liquid may be flashed to lowpressure, e.g., 15 to 50 psia, and rewarmed in the mixed refrigerantsubcooler, if necessary, to efficiently balance the refrigeration loadin the subcooler or to increase the amount of refrigeration produced.

The cooling, condensation, and separation of the feed gas can beoperated advantageously in the range of 150 to 400 psia, and the feedgas can be provided in this pressure range as well. Cooling andcondensation of the feed gas preferably is accomplished by dephlegmationin a dephlegmator, which is a rectifying heat exchanger which partiallycondenses and rectifies the feed gas. Typically a dephlegmator yields adegree of separation equivalent to multiple separation stages, typically5 to 15 stages. Alternatively, cooling and condensation of the feed gasis accomplished in a conventional condenser, defined herein as a partialcondenser, in which a feed gas is partially condensed to yield avapor-liquid mixture which is separated into vapor and liquid streams ina simple separator vessel. A single stage of separation is realized in apartial condenser.

Subcooling of the mixed refrigerant liquid to -175° F. to -225° F. isadvantageous in ethylene plants in order to provide sufficiently coldrefrigeration to cool the feed gas to -170° F. to -220° F., which is thetemperature range required for high (99+%) or ultra-high (99.75+%)ethylene recovery. To attain these high ethylene recoveries, feed gastypically must be cooled to -190° F. to -220° F. in ethylene plantsutilizing conventional partial condensation-type heat exchangers or-170° F. to -190° F. in ethylene plants utilizing dephlegmator-type heatexchangers for final feed cooling.

The bulk of the subcooled mixed refrigerant liquid is split into twoportions. In one embodiment of the invention, one portion of thisrefrigerant is flashed to 15 to 50 psia and at least partially vaporizedin a cold partial condenser or cold dephlegmator to provide the coldestlevel of refrigeration, -180° F. to -230° F., for cooling and condensingthe feed gas. The remainder of the subcooled mixed refrigerant liquid isflashed to 15 to 50 psia and at least partially vaporized in thedemethanizer column overhead condenser to provide reflux to that column.These two mixed refrigerant streams, at about -100° F. to -150° F., arecombined and further warmed and totally vaporized in the warm partialcondenser or warm dephlegmator to provide a warmer level ofrefrigeration for cooling and condensing the feed gas. This warmed mixedrefrigerant vapor stream, typically at -30° F. to -60° F., is mixed withthe stream of mixed refrigerant vaporized in the mixed refrigerantsubcooler, if any, and returned to the mixed refrigerant compressor at15 to 50 psia (preferably 20 to 35 psia).

It is critical to provide refrigeration with the mixed refrigerant cycleat much lower temperatures than the -150° F. level attainable with aconventional ethylene refrigeration cycle, since the amount ofrefrigeration below -150° F. available from other process streams in theethylene plant for feed cooling is limited by operating constraints suchas the amount of high pressure H₂ recovered and the fuel systempressure(s). These constraints limit the amount of expanderrefrigeration which can be produced, which in turn limits the ethylenerecovery. With the mixed refrigerant cycle of the present invention,however, the amount of refrigeration and the coldest temperature levelat which it can be provided are not limited by these constraints, andhigher levels of ethylene recovery can be attained. Additional and/orcolder refrigeration can be provided by the mixed refrigerant cycle byincreasing the amount of low pressure mixed refrigerant which is used tosubcool the high pressure mixed refrigerant liquid. In addition, themixed refrigerant cycle can provide refrigeration colder than the -150°F. level normally supplied to the demethanizer column overhead condenserby an ethylene refrigeration cycle. This colder refrigeration can reducethe amount of ethylene lost in the overhead of the demethanizer columnand further increase ethylene recovery.

With a propylene/mixed refrigerant cascade system, using the closed-loopmixed refrigerant cycle of the present invention, the amount ofrefrigeration and the coldest temperature level at which it can beprovided are not limited by the amount of high pressure H₂ recovered orby the fuel system pressure(s). Therefore, high levels of ethylenerecovery in the present invention can be achieved with much lower feedgas pressures in the range of 150 to 400 psia. The colder refrigerationprovided by the mixed refrigerant cycle can also be used to reduce theamount of ethylene lost in the overhead of the demethanizer column andfurther increase ethylene recovery. In addition, the colderrefrigeration provided by the mixed refrigerant cycle also permits thedemethanizer column(s) to be operated at pressures lower than theconventional 400 to 500 psia level required for high ethylene recoverywhen ethylene refrigeration is utilized as the overhead condenserrefrigerant. At lower pressures, for example in the range of 150 to 400psia, separation of methane and lighter gases from ethylene and heavierhydrocarbons is easier, resulting in lower refrigeration requirementsand lower equipment costs in the demethanizer column system.

This low pressure feed gas chilling concept with a propylene/mixedrefrigerant system of the present invention can also be used to recoverethylene, ethane and/or heavier hydrocarbons from refinery orpetrochemical off-gases. Other refrigerants, such as propane, ammonia orvarious freons, could be used to supply high-level refrigeration inplace of propylene for feed gas precooling and for condensing She mixedrefrigerant. An absorption refrigeration system could also be used tosupplement any of these high-level refrigerants.

If three or more partial condensers or dephlegmators are used in seriesrather than two, the refrigeration for the demethanizer column overheadcondenser could be provided by a mixed refrigerant stream in 10 parallelwith the refrigeration for the intermediate partial condenser(s) orintermediate dephlegmator(s) instead of in parallel with the coldpartial condenser or cold dephlegmator, if the required refrigerationtemperature levels are more closely matched with such an arrangement.Obviously, combinations of one or more partial condensers and one ormore dephlegmators operated in series also could be used. In addition,the demethanizer column overhead condenser could be replaced with adephlegmator or could consist of a dephlegmator operating in series witha partial condenser. In either case, refrigeration for these heatexchangers would be provided by the appropriate mixed refrigerantstreams of the present invention to best match the temperature levels.

The mixed refrigerant cycle also can be used to recover ethylene, ethaneor heavier hydrocarbons from a refinery or petrochemical off-gas. Otherrefrigerants, such as ammonia or various freons, could be used in placeof propane or propylene to supply high level refrigeration for feed gasprecooling and for condensing the mixed refrigerant.

The preferred refrigerant composition will depend upon the specifictemperature levels at which the refrigeration is provided, the systempressure, and the feed gas composition. Representative refrigerantcomposition ranges include 5 to 30 mole % methane, 20 to 55 mole %ethylene and/or ethane and 20 to 50 mole % propylene and/or propane.Lower concentrations of light gases, such as hydrogen or nitrogen andheavier hydrocarbons, such as butane, also may be included.

A first embodiment of the invention can be described in detail withreference to FIG. 1. Warm mixed refrigerant vapor I is compressed to250-500 psia, preferably 300-450 psia, in compressor 101 and cooledagainst air or cooling water in heat exchanger 103 to ambienttemperature. The compressed refrigerant is cooled and condensed,preferably fully condensed, using multiple levels of propane orpropylene refrigeration in conventional refrigeration system 105.Condensed mixed refrigerant 3 at -20° to -50° F. is subcooled in mixedrefrigerant subcooler 107 to yield subcooled mixed refrigerant 5 at-175° to -225° F. The major portion of subcooling is accomplished byindirect heat exchange with cold process streams 7, 8 and 9 from otherparts of the ethylene recovery plant. These cold streams may includework-expanded light gas overhead from the demethanizer column as well ascold methane and hydrogen streams from the hydrogen recovery section ofthe plant. In addition, a portion 11 of the subcooled mixed refrigerant5 may be flashed to 15-50 psia across expansion valve 109 and passedthrough mixed refrigerant subcooler 107 to provide additionalrefrigeration. The flow of refrigerant portion 11 is controlled tobalance the total amount of refrigeration required to subcool the highpressure mixed refrigerant, and to compensate for variations in theproperties of cold process streams 7, 8 and 9. Additional cold processstreams (not shown) can be used to supplement refrigeration from thedescribed cold process streams 7, 8 and 9. Typically about 80 to 100% ofthe total refrigeration for heat exchanger 107 is provided by coldprocess streams and the remainder by flashed subcooled mixed refrigerant13.

A second portion 15 of subcooled mixed refrigerant 5 is flashed to 15-50psia across expansion valve 111 to provide refrigeration at -180° F. to-230° F. for demethanizer column overhead condenser 113 to yield mixedrefrigerant 17 which is at least partially vaporized. A third portion 19of subcooled mixed refrigerant 5 is flashed to 15-50 psia acrossexpansion valve 115 to provide refrigeration at -180° F. to -230° F. asstream 21 for cooling and partially condensing feed gas 23 at pressuresas low as 150 psia. This is accomplished in cold feed condensing zone117 to yield light gas stream 25 and liquid condensate 27 which providesfeed to a demethanizer column (not shown). Preferably cold dephlegmator118 is utilized for feed condensation, which yields several stages ofrectification and thereby reduces the separation duty of thedemethanizer column. Alternatively, a conventional partial condenser canbe used instead of cold dephlegmator 118. Refrigerant streams 17 and 29are combined to form refrigerant stream 31 at about -30° F. to -60° F.,which in turn is combined with vaporized mixed refrigerant 33 from mixedrefrigerant subcooler 107 to provide mixed refrigerant stream 1 at 15-50psia, preferably 20-35 psia, to compressor 101. Any of streams 1, 17,29, 31 or 33 which is partially vaporized can be utilized elsewhere inthe plant to provide additional refrigeration by further vaporization.

An alternative embodiment of the invention is given in FIG. 2 whereinmixed refrigerant is utilized in an additional zone of feed gas coolingand condensation. The mixed refrigerant cycle is similar to that of theembodiment described above for FIG. 1. In this alternative embodiment,refrigeration is provided to demethanizer column condenser 113 and coldfeed condensing zone 117 such that combined mixed refrigerant stream 31is at about -100° F. to -150° F. and is partially vaporized. Feed gas 23is initially cooled and condensed, at pressures as low as 150 psia, byutilizing mixed refrigerant stream 31 in warm feed condensing zone 119to yield warm condensed feed liquid 35 and intermediate vapor stream 37which provides the feed to cold feed condensing zone 117. Preferably,condensing in warm feed condensing zone 119 is accomplished by warmdephlegmator 120. Vaporized mixed refrigerant streams 33 and 39 arecombined to yield mixed refrigerant vapor i to compressor 101. Thetwo-step feed gas cooling and condensing by dephlegmators 118 and 120therefore provide significant prefractionation of the feed into lightgas 25, cold condensed feed liquid 27, and warm condensed feed liquid35. The two condensed feed liquids 27 and 35 can be further fractionatedin single or multiple demethanizer columns of reduced size, sincesignificant prefractionation is provided by the two-step dephlegmatorsystem. Alternatively, feed condensing zones 117 and 119 can utilizeconventional partial condensers instead of dephlegmators 118 and 120, ora combination of a dephlegmator and a partial condenser can be used.

It is possible to utilize the refrigeration system of the presentinvention with more than two dephlegmators or partial condensers inseries as earlier described. In such an alternative, refrigeration at anintermediate temperature level could be provided in parallel to theintermediate dephlegmator(s) or partial condenser(s) and demethanizercondenser 113. Other arrangements are possible to utilize the subcooledmixed refrigerant of the present invention.

EXAMPLE

Energy and material balances were carried out for the embodiment of FIG.2 in which mixed refrigerant vapor stream 1 (3102 lb moles per hour)containing 22 vol % methane, 42 vol % ethylene and 36 vol % propylene iscompressed from -50° F., 24 psia, to 465 psia and cooled to 100° F. withcooling water. Mixed refrigerant vapor 2 is then cooled to -35° F., 455psia, with multiple levels of propylene refrigerant in 105 to totallycondense the mixed refrigerant stream. The mixed refrigerant liquid 3 issubcooled to -200° F. in mixed refrigerant subcooler 107 against thecold H₂, methane, and expander streams 7, 8, and 9 available in theethylene plant. About 1% of the subcooled mixed refrigerant liquid 5 isflashed to 27 psia across expansion valve 109 to yield refrigerant 13which is rewarmed to -38° F. in mixed refrigerant subcooler 107 toefficiently balance the refrigeration load for the system. About 32% ofsubcooled mixed refrigerant liquid 5 is flashed to 30 psia acrossexpansion valve 111, and is partially vaporized and warmed to -125° F.in demethanizer column overhead condenser 113 to provide reflux to thatcolumn. The remaining 67% of subcooled mixed refrigerant liquid 5 isflashed to 30 psia across expansion valve 115, and is partiallyvaporized and warmed to -133° F. in cold dephlegmator 118 to providerefrigeration for cooling and condensing intermediate feed vapor stream37 from -112° F. to -174° F., corresponding to 99.8% ethylene recovery.That is, 99.8% of the ethylene in feed gas stream 23 is condensed andrecovered in the two condensed feed liquid streams 35 and 27, and only0.2% is lost in the light gas stream 25.

Mixed refrigerant streams 17 and 29 are then combined into stream 31which is totally vaporized and warmed to -50° F. in warm dephlegmator120 to provide refrigeration for cooling and condensing feed gas 23 from-33° F. to -112° F. Feed gas 23 (8120 lb moles per hour) containing 24mole % hydrogen, 38 mole % methane, 31 mole % ethylene, 4 mole % ethaneand 3 mole % C₃ and heavier hydrocarbons has been precooled to -33° F.,490 psia by a conventional propylene refrigeration system and otherrefrigerant streams (not shown). Warmed mixed refrigerant stream 39 ismixed with the small stream of mixed refrigerant 33 from mixedrefrigerant subcooler 107 and is returned to mixed refrigerantcompressor 101 at -50° F. and 24 psia to complete the refrigerationcycle.

In this example, the mixed refrigerant-propylene refrigeration systemrequires about 204 less compression power at the same ethylene recoveryof 99.84 than a conventional ethylene-propylene cascade refrigerationsystem to supply the same amount of refrigeration for cooling the feedgas from -33° F. to -174° F. In this example, all of the power savingsis achieved in the propylene compressor, as a result of shifting much ofthe low level refrigerant condensing duty from the lowest and mostenergy intensive level of propylene refrigerant to higher levels.Ethylene refrigerant condenses at a single temperature level, typically-30° F. or -35° F., which concentrates the condensing refrigeration loadin the lowest pressure level of propylene refrigerant. The mixedrefrigerant condenses over a range of temperature, +75° F. to -35° F. inthis example, which spreads the condensing refrigeration load overseveral pressure levels of propylene refrigerant and significantlyreduces the propylene compression power.

With less than a 5% increase in compression power, ethylene recoverycould be increased from 99.84 to 99.94 using the mixedrefrigerant-propylene refrigeration system. This level of ethylenerecovery would not be possible with the ethylene-propylene refrigerationsystem within the operating constraints of the ethylene plant in thisexample.

Thus the refrigeration cycle of the present invention uses a subcooledmixed refrigerant to provide refrigeration at temperatures as low as-175° to -225° F. for the recovery of ethylene at high efficiency andreduced power consumption as compared with prior art technology. Thedistinguishing feature of the invention is that cold process streams andoptionally a portion of flashed subcooled mixed refrigerant are used tosubcool the high pressure liquified mixed refrigerant, which issubsequently flashed to provide very low level refrigeration for feedcondensation and demethanizer column overhead condenser duties. Themethod of the present invention is a significant improvement over theprior art mixed refrigerant cycles earlier described. The mixedrefrigerant cycle described in U.S. Pat. No. 4,072,485 assigned toTechnip is intended to provide low level (below -40° F.) refrigerationin a natural gas processing plant or in the cryogenic section of aconventional (cracked gas) ethylene plant, which employs one or morepartial condensation stages to cool and condense cracked gas feed to thedemethanizer column. In the '485 cycle, the mixed refrigerant is morethan half condensed at near ambient temperature with water or air and istotally condensed at +50° F. with one or more levels of warm propane orpropylene refrigerant. The mixed refrigerant liquid is subcooled to -26°F. with one or more levels of colder propane or propylene refrigerant.In an ethylene plant application, this subcooled mixed refrigerantliquid is then split into two portions. One portion is further subcooledto -b 58° .F in a "secondary" or "auxiliary" heat exchanger against coldprocess streams and the remaining portion is further subcooled to -148°F. in the "main" exchanger against returning low pressure mixedrefrigerant. The two subcooled mixed refrigerant streams are thencombined, flashed to low pressure, and utilized to provide refrigerationover the temperature range of -40° F. to -148° F.; i.e., the mixedrefrigerant is confined to exactly the same temperature range as theethylene refrigeration it replaces. The supply of refrigeration to thedemethanizer column overhead condenser in the ethylene plant is notspecifically addressed in the '485 patent.

A more specific ethylene plant example of the '485 cycle is described inthe earlier-cited article by Kaiser, et al., and indicates a powerreduction of 9% for the '485 mixed refrigerant-propylene system ascompared to a conventional ethylene-propylene cascade system. Thiscompares to a 20% power reduction in the Example for the mixedrefrigerant-propylene system of the present invention. In addition, the'485 example provides for feed gas cooling only to a level of -134° F.,which is not sufficient for a modern high-recovery ethylene plant, anddoes not address the supply of refrigeration to the demethanizer columnoverhead condenser, which would normally require refrigeration at the-150° F. level. With the '485 mixed refrigerant cycle, ethylene recoveryis limited to what could be obtained with the corresponding ethylenerefrigeration cycle, which is well below the 99+% ethylene recovery ofmost modern ethylene plants and far below the 99.75+% ethylene recoveryattainable with dephlegmator-type ethylene plants.

The mixed refrigerant cycle of the present invention is particularlywell-suited to provide low level refrigeration (below-40° F.)specifically in the cryogenic section of an ethylene plant which usestwo or more partial condensation stages, or preferably two or moredephlegmators or combinations of partial condensers and dephlegmators inseries, operating below -20° F., to prefractionate the condensingcracked gas feed before it enters the demethanizer column. In thiscycle, the mixed refrigerant is at least 804 condensed and preferablytotally condensed at -20° F. to -50° F. using one or more levels ofpropane or propylene refrigerant. The mixed refrigerant liquid is thensubcooled to about -200° F. with the major portion of refrigerationprovided by cold process streams. None of the mixed refrigerant issubcooled in the feed gas partial condensers or dephlegmators. The lowpressure mixed refrigerant streams from the cold dephlegmator and thedemethanizer column overhead condenser are then combined and used toprovide refrigeration to the intermediate (if any) and warmdephlegmators or partial condensers. Refrigeration for the demethanizercolumn overhead condenser could alternatively be supplied in parallelwith an intermediate dephlegmator or partial condenser.

In the cycle of the present invention, the mixed refrigerant is nottotally condensed at +50° F. as in the '485 cycle, since this results ininefficient high pressure levels for the mixed refrigerant stream, e.g.,up to 725 psia in the '485 cycle. Instead, the mixed refrigerant of thepresent invention is at least 80% condensed and preferably totallycondensed at -20° F. to -50° F., at pressures below 500 psia. Inaddition, the mixed refrigerant is subcooled in only one heat exchanger,rather than in both the "auxiliary" and "main" exchangers of '485, whichsimplifies operation of the cycle. The mixed refrigerant cycle of thepresent invention also specifically addresses the supply ofrefrigeration to the demethanizer column overhead condenser, whichrequires a significant amount of low level (typically -140° F. to -150°F.) refrigeration.

The mixed refrigerant cycle of earlier-cited U.S. Pat. No. 4,720,293assigned to Air Products and Chemicals, Inc. supplies relatively highlevel refrigeration (+60 to -85° F.) to a single heat exchanger andrelies on vaporization of separated ethane at low partial pressure toprovide intermediate level refrigeration (-85° F. to -170° F.),primarily in the demethanizer column overhead condenser. This requiresthat the separated ethane be combined with the work-expanded H₂ andmethane (which provide the lowest level refrigeration) and which arethen typically sent to fuel after refrigeration recovery. This is highlyadvantageous in processing refinery off-gases when ethane has no valueexcept as fuel, but would not normally be practical in ethylene plants,where the separated ethane has a higher value as feedstock than as fuel,and must be recycled to the cracking furnaces in a relatively purestate.

In addition to the greater power savings and significantly higherethylene recovery provided by the mixed refrigerant cycle of the presentinvention, there are significant capital savings due to thesimplification of equipment with the mixed refrigerant cycle, ascompared to a conventional ethylene refrigeration cycle. For example,the mixed refrigerant compressor of the present invention has only onesuction stream, one suction drum and one recycle control loop. Thetypical ethylene refrigerant compressor has at 1 east three suctionstreams, three suction drums and three recycle control loops, a muchmore expensive arrangement. In addition, the mixed refrigerantcompressor of the present invention, with a suction temperature of -50°F. or warmer, can utilize less expensive metallurgy than the ethylenerefrigerant compressor, which typically has a suction temperature of-150° F. at the first stage of compression. Compared to the conventionalethylene cycle, there are fewer pieces of equipment and lessinterconnecting piping with the mixed refrigerant cycle, resulting inlower overall cost.

With the propylene/mixed refrigerant cascade process of the presentinvention, it is feasible to operate the feed gas chilling train of anethylene plant or other ethylene recovery unit at about 150 to 400 psia,eliminating one or two stages of feed gas compression and eliminatingthe associated capital cost of these compression stages. An equivalentamount of compression energy must be added to the propylene and mixedrefrigerant compressors, but these are incremental increases in thecompression stages, with much smaller associated capital costs. Noadditional refrigeration compression stages are required.

Feed gas chilling, and optionally demethanizing as well, can be carriedout at these lower pressure levels while achieving high ethylenerecovery because the propylene/mixed refrigerant cascade system canprovide all of the necessary refrigeration at temperatures colder than-150° F., regardless of the amount of expander refrigeration available.The propylene/mixed refrigerant system be supplemented with fuel gasexpander refrigeration, but the amount of expander refrigeration is nolonger a constraint to ethylene recovery.

An ethylene recovery unit with a low pressure chilling train operatingin the range of 150 to 400 psia as described in the present inventionprovides additional operating benefits compared with conventionaloperation at higher pressures. These benefits include:

1) less methane and hydrogen are condensed with the ethylene and heavierhydrocarbons, resulting in lower flow rates and lower refrigerationrequirements in the demethanizer column(s),

2) more separation of ethylene and ethane is obtained as the feed gas iscondensed, particularly where one or more dephlegmators are used in thefeed chilling train, resulting in further refrigeration savings in thedemethanizer column(s),

3) more hydrogen can be upgraded in purity to a higher value productsince it is not necessary to expand some of the hydrogen for low-levelrefrigeration, producing a lower value fuel,

4) where a multi-zone demethanizer column system is used, moreseparation of ethylene and ethane in the feed gas chilling section alsoresults in a reduction in the amount of liquid which must be processedin the de-ethanizer column, resulting in a lower flow rate and a savingsin separation energy in the de-ethanizer column,

5) where a multi-zone demethanizer column system is used, moreseparation of ethylene and ethane in the feed gas chilling section alsoprovides more preseparation of the two feed streams to theethylene/ethane splitter column, resulting in a further savings inseparation energy,

6) much of the equipment in the feed pretreatment/drying section, thefeed gas chilling train and, optionally, the demethanizer column(s), isoperated at significantly lower pressure, resulting in reduced capitalcost, and

7) due to the lower pressure ratio between the feed gas and fuel gas,one or more fuel gas expanders are eliminated, further reducing capitalcost.

The closed-loop mixed refrigerant cycle utilized in the low pressurechilling train provides low-level refrigeration (below about -40° F.) inthe cryogenic section of an ethylene plant which uses multiple partialcondensation stages or, preferably, dephlegmators or combinations ofpartial condensers and dephlegmators in series, operating below about-30° F., to prefractionate the condensing cracked gas feed before itenters the demethanizer column system. Compared to a conventionalethylene refrigeration cycle, the mixed refrigerant cycle providessignificant power savings, higher ethylene recovery and also significantcapital savings due to the simplification of equipment. For example, themixed refrigerant compressor has only one suction stream, one suctiondrum and one recycle control loop. The typical ethylene refrigerantcompressor has at least three suction streams, three suction drums andthree recycle control loops, a much more expensive arrangement. Inaddition, the mixed refrigerant compressor, with a suction temperatureof -50° F. or warmer, can utilize a cheaper metallurgy than the ethylenerefrigerant compressor, which typically has a suction temperature of-150° F. at the first stage of compression. Compared to the conventionalrefrigerant ethylene cycle, there are fewer pieces of equipment and lessinterconnecting piping with the mixed refrigerant cycle, resulting inlower overall cost.

The essential characteristics of the present invention are describedcompletely in the foregoing disclosure. One skilled in the art canunderstand the invention and make various modifications thereto withoutdeparting from the basic spirit thereof, and without departing from thescope of the claims which follow.

We claim:
 1. In the recovery of ethylene from a feed gas containingethylene, hydrogen, and C₁ to C₃ hydrocarbons, wherein said recoveryincludes the steps of compressing and cooling said feed gas to condensea portion thereof, fractionating the condensed feed gas liquids in oneor more demethanizer columns to recover a light overhead productcomprising chiefly hydrogen and methane, and fractionating the one ormore demethanizer column bottoms streams to recover an ethylene productand streams containing C₂ and heavier hydrocarbons, refrigeration forsaid recovery is provided in a cycle which comprises:(a) compressing amixed refrigerant vapor comprising two or more components selected fromthe group consisting of methane, ethane, ethylene, propane, andpropylene, and cooling the resulting compressed vapor to yield acondensed mixed refrigerant stream; (b) subcooling said condensed mixedrefrigerant stream by indirect heat exchange with one or more coldprocess streams to yield a subcooled mixed refrigerant; (c) flashing afirst portion of said subcooled mixed refrigerant and using theresulting refrigerant to provide overhead condenser refrigeration for atleast one of said demethanizer columns by indirect heat exchange, whichwarms and at least partially vaporizes said first portion of subcooledmixed refrigerant; (d) flashing a second portion of said subcooled mixedrefrigerant and using the resulting refrigerant to provide at least aportion of the refrigeration required to cool said feed gas by indirectheat exchange and condense a portion thereof, which warms and at leastpartially vaporizes said second portion of subcooled mixed refrigerant;and (e) combining the resulting warmed vapor streams from said first andsecond portions of the subcooled mixed refrigerants of steps (c) and (d)to provide at least a portion of said mixed refrigerant vapor, andrepeating steps (a) through (e);whereby said feed gas is separated intoa vapor stream containing lighter feed components and one or more liquidcondensate streams enriched in heavier feed components, wherein thecooling, condensing, and separating of said feed gas is carried out inthe pressure range of 150 to 400 psis.
 2. The method of claim 1 whereinone of said cold process streams in step (b) is provided by flashing athird portion of said subcooled mixed refrigerant, and wherein theresulting warmed and at least partially vaporized third portion of mixedrefrigerant is combined with the first and second portions of mixedrefrigerant of steps (c) and (d) to provide said mixed refrigerant vaporof step (a).
 3. The method of claim 1 wherein one or more of said coldprocess streams in step (b) is provided by work expanding said lightoverhead product from said one or more demethanizer columns.
 4. Themethod of claim 1 wherein one or more of said cold process streams instep (b) is provided by further cooling and partially condensing atleast a portion of said vapor stream containing lighter feed componentsto produce a hydrogen rich vapor stream and one or more methane-richliquid streams.
 5. The method of claim 1 wherein cooling of said feedgas in step (d) by indirect heat exchange with said second portion ofmixed refrigerant is accomplished by utilizing at least one dephlegmatoror at least one partial condenser or combinations thereof.
 6. The methodof claim 1 wherein said mixed refrigerant vapor is compressed to about250-500 psia prior to cooling and condensing.
 7. The method of claim 6wherein said resulting compressed vapor is condensed at least in part atabout -20° F. to -50° F. prior to subcooling.
 8. The method of claim 6wherein said condensed mixed refrigerant stream is subcooled to betweenabout -175° F. and -225° F.
 9. The method of claim 1 wherein said mixedrefrigerant vapor contains 5 to 30 mole % methane, 25 to 55 mole %ethylene, and 25 to 50 mole % propylene.
 10. The method of claim 1wherein said mixed refrigerant vapor contains 5 to 35 mole % methane, 20to 55 mole % ethane and 20 to 50 mole % propane.
 11. In the recovery ofethylene from a feed gas containing ethylene, hydrogen, and C₁ to C₃hydrocarbons, wherein said recovery includes the steps of compressingand cooling said feed gas to condense a portion thereof, fractionatingthe condensed feed gas liquids in one or more demethanizer columns torecover a light overhead product comprising chiefly hydrogen andmethane, and fractionating the one or more demethanizer column bottomsstreams to recover an ethylene product and streams containing C₂ andheavier hydrocarbons, refrigeration for said recovery is provided in acycle which comprises:(a) compressing a mixed refrigerant vaporcomprising two or more components selected from the group consisting ofmethane, ethane, ethylene, propane, and propylene, and cooling theresulting compressed vapor to yield a condensed mixed refrigerantstream; (b) subcooling said condensed mixed refrigerant stream byindirect heat exchange with one or more cold process streams to yield asubcooled mixed refrigerant; (c) flashing a first portion of saidsubcooled mixed refrigerant and using the resulting refrigerant toprovide overhead condenser refrigeration for at least one of saiddemethanizer columns by indirect heat exchange, which warms and at leastpartially vaporizes said first portion of subcooled mixed refrigerant;(d) flashing a second portion of said subcooled mixed refrigerant andusing the resulting refrigerant to cool and condense an intermediatevapor feed stream by indirect heat exchange in a cold feed condensingzone, which warms and at least partially vaporizes said second portionof subcooled mixed refrigerant; (e) combining the first and secondportions of warmed mixed refrigerant of steps (c) and (d), and using atleast a portion of the resulting combined mixed refrigerant stream tocool and condense said feed gas by indirect heat exchange in a warm feedcondensing zone, which warms and vaporizes said combined mixedrefrigerant stream and provides said intermediate vapor feed stream forsaid cold feed condensing zone of step (d); and (f) returning theresulting vaporized mixed refrigerant stream of step (e) to provide atleast a portion of said mixed refrigerant vapor of step (a), andrepeating steps (a) through (f);whereby said feed gas is separated intoa vapor stream containing lighter feed components, a cold condensed feedliquid enriched in heavier feed components, and a warm condensed feedliquid further enriched in heavier feed components, wherein the cooling,condensing, and separating of said feed gas is carried out in thepressure range of 150 to 400 psia.
 12. The method of claim 11 whereinsaid cooling and condensation in said warm and cold feed condensingzones is accomplished by at least one dephlegmator, at least one partialcondenser, or combinations thereof.
 13. The method of claim 11 whereinone of said cold process streams in step (b) is provided by flashing athird portion of said subcooled mixed refrigerant, and wherein theresulting warmed and at least partially vaporized third portion of mixedrefrigerant is combined with the first and second portions of mixedrefrigerant of steps (c) and (d) to provide said mixed refrigerant vaporof step (a).
 14. The method of claim 11 wherein one or more of said coldprocess streams in step (b) is provided by work expanding said lightoverhead product from said one or more demethanizer columns.
 15. Themethod of claim 11 wherein one or more of said cold process streams instep (b) is provided by further cooling and partially condensing atleast a portion of said vapor stream containing lighter feed componentsto produce a hydrogen-rich vapor stream and one or more methane-richliquid streams.
 16. The method of claim 11 wherein said mixedrefrigerant vapor is compressed to 250-500 psia prior to cooling andcondensing.
 17. The method of claim 16 wherein said resulting compressedvapor is condensed at least in part at -20° F. to -50° F. prior tosubcooling.
 18. The method of claim 16 wherein said condensed mixedrefrigerant stream is subcooled to between -175° F. and -225° F.
 19. Themethod of claim 11 wherein said mixed refrigerant vapor contains 5 to 30mole % methane, 25 to 55 mole % ethylene, and 25 to 50 mole % propylene.20. The method of claim 11 wherein said warm mixed refrigerant vaporcontains 5 to 35 mole % methane, 20 to 55 mole % ethane and 20 to 50mole % propane.